1. Field of the Invention
The present invention relates to electronic detection of improvised explosive devices and underground threats, and more particularly to the use of airborne electromagnetic gradiometers that use synchronous detection to image detonation cables and underground facility wiring and piping.
2. Description of Related Art
Primary electromagnetic (EM) waves will interact with underground devices and infrastructures to create secondary EM waves that are detectable on or above the earth's surface with a gradiometer. The Stolar, Inc. (Raton, N. Mex.) DeltaEM gradiometer survey system provides a tool that can generate subsurface geophysical imaging capabilities with greater sensitivity, range (distance), and flexibility over existing instrumentation. In efforts using local radio sources, EM gradiometry has been shown to be a promising technique. The synchronized EM gradiometer instrumentation is a narrow-band receiver that can discriminate against the spectra noise components and operate in the low ionosphere-earth waveguide noise band, thus maximizing the detection threshold sensitivity of the instrumentation.
An EM gradiometer capitalizes on its high threshold detection sensitivity to secondary EM waves in the ELF/VLF bands. Synchronization to the primary wave in the ELF/VLF bands enables very narrow-band detection with threshold detection sensitivity in the picoTesla (pT) range. Theoretical investigations have found that the secondary EM fields are 20-60 dB below that of the primary EM field components. A significant instrument design issue is the detection of the secondary fields in the presence of the much larger primary field components. This has been solved by the careful design of the gradiometer antennas that achieves 70 dB of primary field suppression.
Two important advantages in underground conductor (UGC) detection have been achieved with this system. First, the magnitude of the scattered secondary wave from the UGC infrastructure increases as frequency decreases. Thus, waves in the ELF/VLF bands have a significant advantage in the UGC detection. Second, the attenuation rate of EM waves in the ELF/VLF bands through soil/rock is very low so that deeply buried structures can be illuminated and detected. The structures may be empty passageways or may contain electrical conductors serving the utility and ventilation needs.
Cellphones were being used to detonate roadside improvised explosive devices (IED's) in Iraq until the US Military countered with radio jamming equipment. Then the Insurgency resorted to stringing long detonation wire pairs that were not subject to radio jamming. This has proved to be difficult to counter. If the IED's or their triggers cannot be disabled as with jamming, then the next best strategy is to detect their deployment and neutralize them before the IED can injure a passing convoy. Primary electromagnetic (EM) waves will interact with surface laid wires and with the wiring and piping in underground infrastructures. Such re-radiate secondary EM waves that are detectable above with an EM gradiometer.
Electrical conductors, such as detonation wires used to trigger IEDs, will scatter low-frequency electromagnetic (EM) waves. These scattered EM waves can be observed with EM gradiometer instrumentation. EM gradiometers can be integrated onto an airframe and used for identifying IED detonation wires from the air when flying ahead of convoys.
Unattended aerial vehicles (UAV's) are presently deployed in theatre for convoy protection. An objective of recent experiments was to optimize the use of an electromagnetic (EM) gradiometer for wire detection from a UAV. Such airborne wire detection technique can provide early warning of the possible threat from IED's several hundreds of feet, to miles ahead of the ground station. Conventional EM gradiometer technology is used by the present inventors for the detection of buried tunnels and conductors, but the equipment is large and manually operated.
As technologies to jam and disable detonation of IED's by mobile cellular telephones are adopted and implemented in theatre, insurgents have resorted to other IED detonation methods, the most obvious of which is by electric wire detonation. A detonation wire pair, usually fairly long in length, is used to connect the IED to the person who performs the detonation. These wires are conductive and if stimulated with a primary electric field, reradiate a secondary field which can be used to identify the “threat” and to determine its location. Extensive engineering investigations and field tests have confirmed that electrical conductors scatter low-frequency electromagnetic (EM) waves. These scattered EM waves can be observed with an EM gradiometer instrumentation. Presently the technique has only been demonstrated on the ground but the EM gradiometer can also be integrated in an UAV, as illustrated in FIG. 1 thereby allowing investigation of the land ahead of a convoy and reporting any indication of a threat from detonation wires in real time. The UAV carries both visible and IR cameras and a Global Positioning System (GPS), locating exact coordinates for the region of interest.
The source of the primary EM wave is a vertical magnetic dipole (loop antenna) mounted on the lead vehicle in the convoy. The loop antenna generates omni-directional toroidal EM field components. The primary electric field (EP) lies in a horizontal plane, as illustrated in FIG. 1. There is a magnetic field (HP) component. When the primary electric field (EP) component illuminates the IED detonation wire pair, the induced current flow (I) in the detonation wire can be approximately determined from the long wavelength scattering limit of mathematical physics given by;
                    I        =                                            2              ⁢              π              ⁢                                                          ⁢                              E                P                                                    ωμ              ⁢                                                          ⁢                              Log                ⁡                                  (                                      κ                    ⁢                                                                                  ⁢                    a                                    )                                                              ⁢                                          ⁢          amperes                                    (        1        )            where κ=β−iα; β is the phase constant and α is the attenuation rate,
ω=2πf and f is the operating frequency in Hertz,
a=radius of the detonation wire pair, and μ=μrμo is the magnetic permeability.
The above equation (1) shows that the induced current increases as the operating frequency is reduced. The induced current flow produces a cylindrically spreading secondary wave that is observable by a low-flying UAV. The secondary magnetic field component is given by
                              H          S                =                              I            2                    ⁢                                    (                                                ⅈ                  ⁢                                                                          ⁢                  κ                                                  2                  ⁢                  π                  ⁢                                                                          ⁢                  r                                            )                                      1              /              2                                ⁢                      ⅇ                                          -                ⅈκ                            ⁢                                                          ⁢              r                                                          (        2        )            where r=the radial distance in meters from the detonation wire to the UAV. An important aspect of the secondary magnetic field component is that it decays in magnitude by only the half power of distance (r) from the detonation wire.